I. Field of the Invention
This invention relates to the field of communication systems and, in particular, to maximize the use of available capacity in a communication system where signals associated with multiple users may be simultaneously transmitted on a common channel.
II. Description of the Prior Art
Telecommunications traffic can be divided into a number of classes. One classification scheme divides the traffic based upon the rate at which the traffic is transmitted and the priority of the traffic. In accordance with this classification scheme, traffic is classified as constant bit rate (CBR) traffic, variable bit rate (VBR) traffic, or available bit rate (ABR) traffic. (CBR) traffic is afforded a fixed bit rate regardless of the requirements of the data that is to be transmitted. This is the most expensive type of service available. VBR traffic allows a user to decide the rate at which the traffic is sent for each communication. ABR traffic is the lowest priority traffic. ABR traffic is transmitted at whatever rate is available. Accordingly, ABR service is relatively inexpensive.
One example of traffic that is best sent using CBR service is conventional fixed rate circuit switched traffic. Examples of signals having the variable demands suitable for VBR service are speech and Internet video services. Both CBR and VBR traffic are usually real time with a relatively high quality of service requirement. The quality of service is an indication of the reliability that data will be successfully received as well as the delay involved in the reception. ABR traffic has a lower priority and does not provide high probability that the traffic will be delivered within a short time interval. Traffic suitable for ABR service includes file transfers and electronic mail transfers. If loading is not high, and delay is therefore not high, most world wide web transmissions use ABR service.
The forward link capability of a cellular communication system (i.e., the number of users and the bit rate of each user) is in part controlled by the capabilities of the power amplifier used to amplify the signals transmitted from the base stations of the system. For example, in a code division multiple access (CDMA) communication system, each of the traffic streams transmitted is assigned to a code channel. Details of an exemplary CDMA system can be found in U.S. Pat. No. 4,901,307 entitled "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters", which is assigned to the assignee of the present invention and incorporated herein in its entirety by reference. Each channel in a CDMA system is modulated over a frequency band (which is the same for each code channel) and combined to form a CDMA channel. The amount of power required in each code channel depends upon the bit rate of traffic transmitted over that code channel, the gains of the antennas at the receiving station (such as a mobile station) and a transmitting station (such as a base station), the path loss (i.e., the amount of attenuation of the signal) between the base station and the remote station to which the information is sent, the noise level at the mobile station, and the performance of the modulation scheme used. The noise level at the mobile station includes thermal noise, noise from other cells that the mobile station is not receiving, and noise from non-orthogonal signal components from the cell that the mobile station is receiving. The CDMA channel is amplified by the power amplifier within the base station. The base station must transmit a total power sufficient for an intended receiving mobile station to receive the signals directed to it at the desired error rates. The base station uses various procedures so that the total amount of power required by the CDMA channel does not exceed the amount of power that the power amplifier can provide without undesirable distortion.
The forward link capability of a cellular communication system is also limited by the amount of interference from the user's own cell (from non-orthogonal components if the waveform is transmitted orthogonally as in TIA/EIA-95) and by the interference from signals transmitted by other cells. This provides a limit irrespective of the amount of power that the base station transmits. In this situation, increasing the base station's transmission power above some limits only marginally increases the capability of the system.
The maximum output power level of a base station is determined by a number of design parameters related to the power amplifier of the base station. Two relevant parameters of the power amplifier include power dissipation and unwanted emissions. Unwanted emissions are emissions that are outside the bandwidth of a transmitted signal. A large portion of the unwanted emissions occur due to intermodulation within the power amplifier. Intermodulation is a form of distortion. Intermodulation distortion increases as the power amplifier is driven closer to the maximum output of the amplifier. Regulatory bodies, such as the Federal Communication Commission often limit unwanted emissions. Industry standards can also set limits on unwanted emissions in order to avoid interference with the same system or another system.
In order to maintain unwanted emissions within the required limits, the output power capability of a power amplifier is selected to provide a very small probability that the unwanted emissions will exceed the required limit. When the requested power exceeds the maximum output power, a base station can limit the output power in order to maintain the unwanted emissions within the prescribed limits. However, the demand on the power amplifier is determined by the number of traffic streams that are transmitting at the same time. Each transmitted traffic stream can start and end arbitrarily. Therefore, it is difficult to determine the amount of power that the base station is required to transmit at any particular time.
An important measure in a communication system is the signal-to-noise ratio. In a digital communication system, the required signal-to-noise ratio is equal to the product of the bit rate and the required energy per bit divided by the total noise spectral density. The error rate of the communication system is often expressed in terms of the bit error rate or the frame error rate. The error rate is a decreasing function of the signal-to-noise ratio. If the received signal-to-noise ratio is too low, then the probability that an error will occur is very high. Thus, a communication system attempts to maintain the received signal-to-noise ratio at or above the required signal-to-noise ratio for the desired error rate.
Accordingly, in mobile radio communication systems such as CDMA systems, where multiple users simultaneously transmit on a common channel, the number of simultaneous VBR and CBR users permitted within telecommunication system is usually limited. The limit is selected to maintain a low probability of exceeding the maximum output power. When selecting the limits on the number of users, the variable rate nature of the VBR services and the dynamic power control on the forward link must be considered.
While the characteristics set forth above have been described in connection with the forward link, similar characteristics also apply to the reverse link.